JPS6137339A - Fatigue-resistant bolt and its production - Google Patents

Abstract

PURPOSE:To increase the surface hardness and hardening depth of a screw bottom and to obtain a fatigue-resistant bolt by improving the conditions for rolling the bolt consisting of a middle-carbon steel or low-alloy steel. CONSTITUTION:A bolt blank 1 is formed by using the middle-carbon steel or low-alloy steel as a blank material. The blank is then subjected to a heat treatment for hardening and tempering and is heated to a 250-350 deg.C range. The blank is thereafter subjected to thread rolling under the conditions of >=6m/sec biting speed of rolling dies right after the heating. The bolt 4 obtd. in the above- described manner has >=0.075 ratio of the surface hardened layer thickness with respect to 1/2 root diameter and >=1.15 increase rate of the surface hardness with respect to the axial center. The fatigue strength of the bolt is thus remarkably improved and the bolt 4 having excellent thread accuracy is obtd.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention provides a method for manufacturing medium carbon steel or low alloy steel bolts by thread rolling, and more specifically, a method for manufacturing medium carbon steel or low alloy steel bolts by thread rolling. The present invention relates to a fatigue-resistant bolt with improved strength as much as possible and a method for manufacturing the same.

(Conventional technology) Bolts using medium carbon steel or low alloys.

It is well known that cold thread rolling of heat-treated layers increases fatigue strength. This is said to be because compressive residual stress is generated at the thread bottom due to thread rolling, and a hardened surface layer is formed. Of this.

The surface hardening layer is related to the surface hardness and hardening depth, and if the surface hardening coefficient, which is related to the difference in hardness between the surface hardness and the shaft center, is dr, the root diameter of the bolt is dr, and the hardening depth is tr, then ,
It is assumed that the following relationship approximately holds true between the fatigue strength increase rate H of steel bolts (fatigue strength of surface hardened bolts 7 fatigue strength of bolts without surface hardening) .

Therefore, it can be seen that in order to improve the fatigue strength of a steel bolt, it is sufficient to increase the surface hardening coefficient in the above formula and the hardening depth tr.

(Problems to be Solved by the Invention) However, in JA construction of bolts using the conventional cold rolling method, there is a certain limit to the increase in the surface hardening coefficient and the hardening depth tr. The current situation was that it was not possible to increase the fatigue strength of bolts made of steel or low-alloy steel as desired.

In view of the above-mentioned current situation, the present invention attempts to improve the rolling method and obtain a bolt with excellent fatigue resistance.

(Means for Solving the Problems) Therefore, the present invention involves forming a bolt preform from medium carbon steel or low alloy steel, then subjecting it to quenching and tempering heat treatment, and then heating it to a temperature of 250 to 350 The rolling die is heated to a temperature range of 6 trrm.
Thread rolling is now performed under conditions of /see or higher.

With this X, 250
The reason for heating to a temperature range of ~350°C and performing thread rolling directly is to increase the surface hardening coefficient K and hardening depth tr in the above formula as much as possible to improve fatigue strength. It's for a reason.

That is, it is inferred that when initial martensite is processed, many vacancies, which are lattice defects, are formed, and carbon atoms gather in these holes when heated. On the other hand, it is expected that work hardening will occur more easily if the material is processed near the temperature at which the high-temperature tensile strength characteristic of the material reaches its maximum value. The present invention aims to increase the surface hardening coefficient K as much as possible through the synergistic effect of the above two phenomena, and the effect is greater when the heating temperature (rolling temperature) is lower than 250°C or higher than 350°C. Since the temperature becomes small, this temperature was limited to 250 to 350°C.

Further, when processing is performed at a higher temperature, the flow area of the material at the thread bottom is expanded due to a decrease in deformation resistance, and as a result, the hardening depth tr increases.

Furthermore, the heating temperature range of 250 to 350°C corresponds to the blue brittle region. However, it is known that when high-speed machining is performed, the blue brittle region shifts to the high temperature side, and in the present invention, the machining speed, that is, the biting speed of the rolling die, is set to
The blue heat brittle region was made the same size by limiting the range to be equal to or larger than .

And in the bolt obtained as above, the ratio is 1.1
5 or more. As a result, the fatigue strength of the bolt is significantly improved not only for bolts obtained by heat-rolling, but also for bolts obtained by cold-rolling in heat-treated layers.
Moreover, a bolt with excellent thread accuracy can be obtained.

(Example) An embodiment of the present invention will be described below with reference to the accompanying drawings.

Implementation 12 I Made from JIS 80M435 wire rod (polished product).

This was cut to a fixed size, and then cold forged to form a bolt head, as shown in Figs. 1(a) and (b).

Bolt basic product 1 was obtained. The bolt blank 1 includes a shaft portion 2 on which a thread will later be formed, and a dish-shaped head portion 3.

Next, the bolt blank 1 was heat-treated under the conditions of oil cooling at 850°C and then oil cooling at 540°C. The mechanical properties of the heat-treated layer were as follows: tensile strength: 120 Kj'/mmζ hardness: HRC35.

Next, the bolt product 1 that has undergone the heat treatment is heated to various temperatures from normal temperature (room temperature) to 600° C. using a high-frequency heating device, and at the same time as the heating is finished, it is transferred to a flat die rolling machine and rolled into a rolling die. Thread rolling is performed at a biting speed of 6.7 tan/sec, and the rolled bolt is then dropped into oil.
Cooled. Figure 2 shows the dimensions and shape of the bolt obtained as described above.On the side opposite to the head of the bolt, indicated by 4, there is a predetermined distance (20 mm in this case) and a predetermined shape. In this case, after the screw 5 of MgF2.25) is molded, the bolt 4 is subjected to a hardness test to measure the surface hardness and hardening depth of the screw bottom, and in parallel, a fatigue test and an impact test are performed. It was subjected to tests to measure fatigue limits and impact strength. The hardness test was conducted using a Vickers hardness tester, the fatigue test was conducted using a strong vibration tester, and the impact test was conducted using an impact tensile tester.

It shows the relationship of σa ("J'/rrrrtt"). In addition, in the same figure, the results of heat treatment after cold rolling as a comparative example are also shown as P.

This is it, the fatigue limit σa is the surface hardness H and the hardening depth t'r.
The value tends to increase as both increase. When processing is carried out in the rolling temperature range of 250 to 350°C, which is the range of the present invention, the fatigue limit σa is much higher, and the value is slightly less than twice that of the one rolled at room temperature (R2T), after rolling. It is more than 2.5 times that of the cold rolled product (P). Furthermore, the impact strength E rapidly decreased at a rolling temperature of 500°C, confirming that the blue brittle region had shifted to the high temperature side.

The ratio of the surface hardening layer depth to the A valley diameter of the bolt processed at a rolling temperature range of 250 to 350°C is 0.0.
It is about 9. Also, as an example, the rolling temperature is 300
The surface hardness distribution of bolts treated at ℃ shows a button pattern as shown in Figure 4.

In this case, the increase rate of the surface hardness relative to the internal hardness is about 1.2.

Example 2 JIS 45C wire rod (polished) material was used as the raw material, and the bolt strength was 87 TiP/fin 1.0 as shown in FIGS. 1(a) and 1(b) in the same manner as in Example 1. The hardness was HRC24.

Subsequently, the preformed product 1 that has undergone the heat treatment is heated from room temperature to 600°C.
The bolt 4 (see FIG. 2) was obtained by heating to various temperatures up to .degree. C., and then subjected to the same treatment as in Example 1, and then subjected to the same tests as in Example 1.

Figure 5 shows the test results, and the fatigue limit σa
As in the case of SCM 435, the value tends to increase as both the surface hardness H and the hardening depth tr increase. When processing is performed at a rolling temperature of 250 to 350°C, which is the range of the present invention, the fatigue limit σa is
), which is approximately 1.5 to 23 times that of cold rolled after heat treatment (
The value was approximately 4.5 to 7 times that of P). Furthermore, impact strength E
It was confirmed that when the rolling temperature exceeded 500°C, the temperature rapidly decreased, and the blue brittle region shifted to the high temperature side.

The ratio is about 0.10 during the rainy season. As an example, the surface hardness distribution of a bolt treated at a rolling temperature of 300'q is shown in FIG. From this, the rate of increase in hardness of the narrow surface relative to the inside is approximately 1.15.

Regarding 80M435 and 8450, Example 1 is provided separately.
When the same heat treatment as in Example 2 was carried out and a high-temperature tensile test was conducted, the high-temperature tensile strength of both cases reached the highest value at around 300° C., as shown in FIG.

This temperature is within the heating temperature range of 250 to 350 in the present invention.
It is believed that this was an effective move to improve the fatigue limit.

(Effects of the Invention) As explained above in detail, the present invention first heat-treats the Porto molded product, and then heats it to 250 to 350°C to perform thread rolling. The surface hardness and hardening depth of the hardened surface layer at the bottom of the screw thread have increased, making it possible to obtain a bolt with better fatigue resistance than ever before.

[Brief explanation of the drawing]

, 'gi Figures (a) and (b) are a front view and a side view showing the dimensions and shape of the bolt material obtained in the intermediate process of the method of the present invention, and Figure 2 is a completed bolt fitted according to the present invention. Figure 3 is a side view showing the dimensions and shape of the invention.
Figure 6 is a correlation diagram showing the surface hardness distribution of 545C bolt rolled at 300℃, Figure 7 is a correlation diagram showing the high temperature tensile properties of 80M435 and 545C. be. 1... Bolt original product 4... (Completed) Bolt Figure 1 Figure 2 Figure 7 Figure 3 Turning temperature ('C) Figure 4 Distance between screw jFS force) (mm) Figure 5 Figure 6 Distance between screw claw force (mm)

Claims (2)

[Claims] (1) A bolt in which a threaded portion is formed by warm rolling using medium carbon steel or low alloy steel as a material, wherein the ratio of the surface hardened layer thickness to the 1/2 root diameter of the threaded portion is 0.
075 or higher and the rate of increase in surface hardness relative to the axis is 1
．． A fatigue-resistant bolt characterized by having a resistance of 15 or more. (2) Form a bolt shape from medium carbon steel or low alloy steel, then heat treat it by quenching and tempering, then heat it to a temperature range of 250 to 350°C, and immediately after that, roll it into a rolling die. A method for manufacturing a fatigue-resistant bolt, characterized in that thread rolling is performed under conditions of a biting speed of 6 mm/sec or more.